In recent years there has been considerable interest in optical communication systems. Such interest has been enhanced by recent advances in the development of a variety of optical circuit devices including low loss optical fibers suitable for long range transmission of light waves and cheap efficient semiconductors for use as signal transmitters.
Among the advantages of optical communication systems over conventional telecommunication systems are a greatly increased information carrying capacity inherent in the high frequency of light waves, as well as the small size, rugged construction and low cost of such systems. The channeling of optical energy through waveguides can be accomplished by surrounding or bounding an optically transparent material, such as glass with another material of lower index of refraction such as air or another type of glass. The reason light is efficiently confined and transmitted in such a waveguide is based on the physical principle that light transmitted through the material having the higher index of refraction is totally reflected at the boundary formed with the lower index of refraction material, provided that the angle of incidence is less than a predetermined angle. This can produce a light guiding effect as a beam traveling through the higher index material at an oblique angle to the boundary undergoes successive total reflections at each boundary.
Multimode propagation, each mode being characterized by its own electric field profile transverse to its direction of propagation, occurs in optical waveguides of thickness greater than approximately one wavelength, i.e., one micrometer for near-infrared light, whereas single mode propagation occurs in waveguides having a thickness on the order of a wavelength of light. From the standpoint of cost, difficulty of fabrication, and efficient transfer of light into and out of connecting optical fibers, the preferred thickness range of optical waveguides in most optical communication systems is from 20 to 200 micrometers.
Therefore, many, if not most, prospective optical communication systems will operate with multimode propagation.
In a fiber optical communication system, it is desirable that a plurality of information carrying light beams of different wavelengths be combined (multiplexed) onto a single fiber to increase its information carrying capacity. At the receiving station, the beams are spatially separated (demultiplexed) and each beam received by a separate detector for recovery of the information that it carries. In one prior art approach, such multiplexers have been constructed of bulky and hard-to-align discrete components such as lenses and frequency selective components, such as prisms, diffraction gratings, or dielectric filters.
In another approach, a diffraction grating is formed within an optical waveguide by spatially modulating either the thickness of the index of refraction of the waveguide to create grating lines. By varying the spacing of the grating lines along the direction of propagation of the light, different component wavelengths of a wavelength multiplexed light beam are reflected at different places along the waveguide. An example of this approach may be found in U.S. Pat. No. 3,814,498 by W. J. Tomlinson III, entitled, "Integrated Optical Circuit Devices Employing Optical Gratings," issued June 4, 1974. Because the approach is limited to waveguides of no greater than several micrometers in thickness, it is not practical for use in multimode systems which, as discussed above, typically use waveguide thicknesses greater than 20 microns. Another disadvantage of this approach is that the grating, being within the waveguide, is difficult to fabricate.
Another approach to the wavelength multiplexing and demultiplexing of optical systems is provided in an article by K. Kobayashi and M. Seki, entitled, "Micro-Optic Grating Multiplexers for Fiber-Optic Communications," Optical Fiber Communication Conference, Mar. 6-8, 1979, Washington, D.C., Technical Digest pps 51-57. In this approach, a reflective diffraction grating is replicated onto one end of a graded-index (diametrically graduated index of refraction) rod, and an input-output optical fiber array is attached to the other end of the rod. A light beam having multiple wavelength components diverges from an input fiber, is transformed into a parallel beam by the graded-index rod and is diffracted by the grating at angles according to the wavelength of the components. Each of the diffracted beams is then focussed by the graded-index rod onto separate output fibers. While capable of propagating multimode beams, an inherent limitation of the graded-index rod is that it can focus light well only over a narrow wavelength region and is thus not suitable for use with multiple semiconductor laser sources operating over a wide range of wavelengths.